Biological motion recognition in point-light displays exhibits a well-known inversion effect (Grossman and Blake, 2001; Pavlova and Sokolov, 2000) - analogous to inversion effects in other expert recognition systems like face recognition (Yin, 1969). We measured human observers' sensitivity to perturbations of intermediate level spatiotemporal features in point-light displays of upright and inverted walkers. We found that observers are more sensitive to perturbation of certain features in upright displays but more sensitive to other features in inverted displays. The features with greater sensitivity in upright displays describe the relative motion of adjacent limb segments (e.g., left thigh/right thigh, and thigh/leg). In contrast, perturbations to a feature that describes the angular velocity of the limbs result in greater sensitivity in inverted displays.

We hypothesize that certain intermediate level features are used by the visual system for biological motion recognition (Casile and Giese, 2005). Detection of these features may be orientation sensitive and therefore observers more sensitive to perturbations of these features in upright displays. On the other hand, if a feature is not used for recognition, the sensitivities may be the same under upright and inverted conditions, and may actually be higher in inverted displays because of greater impact of local attentional mechanisms in the absence of global high level integration.

We thank the Center for Human Modeling and Simulation for use of the ReActor system. This research was supported by the DoD Multidisciplinary University Research Initiative (MURI) program administered by the Office of Naval Research under grant N00014-01-1-0625.